Returns an integer string giving the current access position in
channelId. This value returned is a byte offset that can be passed to
seek in order to set the channel to a particular position. Note
that this value is in terms of bytes, not characters like read.
The value returned is -1 for channels that do not support
seeking.

ChannelId must be an identifier for an open channel such as a
Tcl standard channel (stdin, stdout, or stderr),
the return value from an invocation of open or socket, or
the result of a channel creation command provided by a Tcl extension.